The overall goal of this study is to determine the biological significance of a weak, but specifically directed, anti-tumor immune response to primary tumor development. Highly immunodepressed histocompatible mice obtained either by sublethally X-irradiating mutant athymic nude mice or by thymectomizng, lethally X-irradiating, and bone-marrow repopulating adult non-mutant mice will then be immunomodulated by infusing them with various numbers of specifically sensitized lymphoid cells. The first approach will use autochthonous hosts where the tumors will be induced with either murine mammary tumor virus (MTV) or 3-methylcholanthrene (MCA). These immunomodulated mice will be used to determine whether a weak, but specifically directed, anti-tumor immune response will induce a higher than normal probability of tumor development over that seen in mice with an immune responsiveness that is either much stronger or essentially absent. The tumors induced in these immunologically modulated mice will be tested to determine whether the immunologic status of the host was able to affect the immunogenicity of the resultant tumors. The second approach will determine whether a weak, but specifically directed, immune response will stimulate the progression of transplantable neoplasms towards malignancy.